The use of a laser with electron microscopes and focused ion beam (FIB) systems provides a variety of applications in the field of manufacturing, repair, and inspection of miniature devices, such as integrated circuits, magnetic recording heads, and photolithography masks. The applications can also be used in the fields of energy exploration, material sciences, and biology. The electron microscope, typically a scanning electron microscope (SEM), provides high-resolution image with minimal damage to the target, or workpiece, and the FIB is used to alter the workpiece and to form images. Other types of electron microscopes, such as transmission electron microscopes (TEM) and scanning transmission electron microscopes (STEM) can also be used.
The usage of both a FIB with a SEM/STEM/TEM can provide the ability to image inside of a specimen. FIB systems operate in a similar fashion to a scanning electron microscope except, rather than a beam of electrons and as the name implies, FIB systems use a finely focused beam of ions (usually gallium) that can be operated at low beam currents for imaging or high beam currents for site specific imaging, deposition, or milling. One common usage of FIB tools is to machine surfaces. An ideal FIB machine might be able to machine tens or hundreds of microns into a sample with high beam currents and acceleration potentials. With low beam currents and acceleration potentials, it may be able to machine away layers approaching atomic thicknesses without damaging layers that are nanometer scale or even less. Such micro- and nano-machining capability of the FIB is often very effective when used with the SEM/STEM/TEM for high resolution imaging. In other processes, a FIB could be used to cut unwanted electrical connections, and/or to deposit conductive material in order to make a connection in a semiconductor. FIBs are also used for maskless implantation. In these FIB and/or SEM processes, it is often needed to create an environment that is relatively free from obstructions. Unfortunately, a number of different processes, including the bulk material removal processes, will create large amounts of debris that can potentially interfere with the SEM/STEM/TEM and FIB.
A use of a laser has recently been introduced in this field for rapid processing of the workpiece. For example, U.S. Pat. Pub. 2011/0163068, by Utlaut et al., for “Multibeam System” filed Jan. 9, 2009, is directed to a FIB system that uses an additional laser beam for rapid processing. When used together with a SEM/STEM/TEM and FIB system, the process of laser ablation can remove materials from surfaces by irradiating it. Very fast picosecond lasers perform an athermal ablation process while slower nanosecond and continuous wave lasers (“CW lasers”) provide thermal ablation processes.
Usually, laser ablation is performed using a pulsed laser. As compared to charged particle beam processing, laser ablation is capable of removing a relatively massive amount of material very quickly. The wavelength of lasers, however, is much larger than the wavelength of the charged particles in the charged particle beams. Because the size to which a beam can be focused is, in part, limited by the beam wavelength, the minimum spot size of a laser beam is typically larger than the minimum spot size of a charged particle beam. Thus, while a charged particle beam typically has finer resolution than a laser beam and can micromachine an extremely small structure, the beam current is limited and the micromachining operation can be unacceptably slow. Laser micromachining, on the other hand, is generally much faster, but the resolution is inherently limited by the longer beam wavelength. Other uses of laser machining are described, for example, in U.S. Pat. No. 8,168,961, by Straw et al., for “Charged Particle Beam Masking for Laser Ablation Micromachining,” which is assigned to the assignee of the present invention and is hereby incorporated by reference. U.S. Pat. No. 8,168,961, is not admitted to be prior art by its inclusion in this Background section.
Recent advancements allow the use of lasers for careful preparation of workpieces so that they can be used for a SEM/STEM/TEM and FIB system. SEM/STEM/TEM and FIB tools require prepared samples with thin heat affected zone (“HAZ”) areas, and the system may target one extremely precise location within a large sample, such as a submicron scale defect. It is generally not known how to use laser techniques to cut bulk samples while at the same time being able to produce thin HAZ and/or micron scale end pointing that is suitable for SEM/STEM/TEM and FIB processing.
There are some instances when different types of lasers are preferred. Some lasers are best for large bulk material removal from workpieces. For example, these lasers can cut packaging materials or cut out large unnecessary materials from the workpiece so that the workpiece is of appropriate size for analysis. They can also be used for cutting deep trenches in a workpiece, so that an area for study can be exposed. Lasers employed for cutting typically range in power from watts to hundreds of watts. Pulse widths ranging from a nanosecond up to continuous wave are common. Longer wavelengths (e.g., common visible and near IR wavelengths), such as 532 nm, 1064 nm, and up to 10.6 microns (e.g. CO2 lasers) are often used. These lasers provide high laser fluence (pulse energy divided by irradiated area) to a workpiece, exceeding the ablation threshold to produce a range of features including holes and trenches. The high powers used in these rapid cutting processes tend to cause a heat affected zone that must be removed prior to performing fine SEM/STEM/TEM and FIB processes such as imaging or circuit editing. Debris incompatible with the vacuum environment of the FIB and SEM/STEM/TEM may also be produced by the high material removal rates.
Different types of lasers are better suited for precision work such as micro-machining, drilling, routing, trenching, etching, and heat affected zone removal processes. Common lasers for this type of work employ shorter wavelengths, shorter pulse widths, and more tightly focused spots than lasers used for bulk material removal. While lasers up to micron-scale wavelength can be used, short wavelength lasers such as 355 and 266 nm are preferable because many materials readily absorb these wavelengths and they can be focused to small spot sizes. Short pulse width lasers such as tens of picoseconds and down into the femtosecond regime are also preferred because they provide material removal with a small heat affected zone. Typical lasers employed for these applications will have power levels on the order of watts or less.
There are numerous samples composed of disparate materials which may be desirably processed by lasers prior to imaging or alteration with an SEM/STEM/TEM and/or FIB. Some examples of these samples include semiconductor devices such as chips or wafers, packaged chips including 3D stacked packages that contain semiconductor materials, wires, and bonding agents, lithography masks, biological samples, mineral samples such as rocks, or material samples such as composites, ceramics, glasses, coatings, glues, rubbers, polymers, superconductors, magnetic materials, alloys, and metals. This is a list of a few samples representing the diversity of workpiece materials which may be processed. Many other sample materials which are not listed may also be processed.
Lasers are often selected for advantageous processing of structures of different materials. For example, a 10.6 micron wavelength CO2 laser may be preferred for cutting through the packaging materials of a 3D stacked package, a pulsed 1.3 micron wavelength laser may be preferred for cutting metallic structures on the semiconductor chips within the package, and a short-wavelength pulsed laser may be preferred for processing the silicon structures on the chip. Because silicon is transparent to light of a wavelength of about 1100 nm or longer and metals absorb IR wavelengths, processing can occur with less damage to the silicon.
One challenge is that a system with a single laser cannot perform all the desired functions when different laser beams are necessary. In other words, there is no known way of performing the multiple tasks associated with different types of lasers without having to separately buy and use multiple systems configured with different laser types. In these cases, the workpiece would undergo one laser treatment from one system, transported to another laser treatment system, and then undergo another laser treatment.
Another challenge is that undesirable debris from the machining process may become deposited upon critical components in the FIB and/or SEM/STEM/TEM chamber, such as sources and detectors, and degrade their efficacy. This type of debris has the potential to hinder the functions of the FIB/SEM/STEM/TEM microscopy. Conventional electron microscopes also generally require samples to be imaged under vacuum because a gaseous atmosphere rapidly spreads and attenuates electron beams. As a consequence, the debris expelled during laser machining processes can degrade the vacuum environment and impede imaging. In other cases, the unnecessary debris after the machining process may be so large that it prevents the workpiece from actually fitting into the vacuum chamber of the SEM/STEM/TEM.
What is needed is a system wherein multiple types of laser beam spots can be used to process a workpiece in conjunction with SEM/STEM/TEM and FIB without producing large amounts of ablated material in the processing chamber.